EXHAUST BACKFIRE

Discussion in '94-95 5.0 - Specific' started by 99GreenStang, Jan 4, 2014.

  1. 99GreenStang

    99GreenStang Well-Known Member

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    Hey fellow members as most of you know or realize I have a 99 mustang with a 302 ( 308) that is carb , well ive battled every issue possible with this car and I lay money I have had more problems then anyone on such a motor . Im running a msd mechanical advance distributor and I have been told that vacumn advance distributor would be the best for street , but I have tried tuning the mechanical to make due with it , today i realized i had way to much timing I was running 18-20 degrees intial and 18 degrees advance so I had way to much initial . I switched the spring in the distributor to a light blue and a light silver and the bushing is now a silver . inital timing is set at 8 degrees , firing order is the 154 method its a 1974 block with a custom grind cam and I have recently been told that maybe I need to run the H.O firing order instead ?? the exhaust backfire seems to only come from the driver side but whats weird is I adjusted this springs and bushing and set timing correct and the carb was adjusted wack because of the timing being messed up before . Im running a Road Demon 625cfm vacum advance carb its 4 corner idle and all screws are out half a turn right now and the car was running at 8-1000 rpm and was counting the licks off ! sounded better than it ever has ! then i left came back and started if up the engine was cold and the popping and backfire started again!! how could it run perfect and then start that with nothing being changed ? my camshaft is a custom grind 280 duration with a 480 lift . also i looked at my plugs and they have very heavy black carbon on them proble because of the very rich condition the motor was running before ! ANY IDEAS PEOPLE IM READY TO PULL MY HAIR OUT LOL!
     
  2. nightfire

    nightfire Well-Known Member

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    This is precisely why no one does these swaps. Best of luck.
     
  3. 99GreenStang

    99GreenStang Well-Known Member

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    No one does these swaps because a exhaust backfire lol ? It runs ! It moves ! It looks awesome ! I don't see the point in what you say other than it's a theory and I understand it's a car and there known to have problems that's why we have help forums . My main question is with a custom grind cam in a 1974 block do I need to use the h.o firing order ? Any ideas what else it could be ? Not enough timing or too much etc ? It just puzzled me it ran fine with the same settings then I left and came back and it's back to messing up .
     
  4. Brandon123

    Brandon123 Active Member

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    Yes, if you have a custom cam use the HO firing order...
     
  5. 99GreenStang

    99GreenStang Well-Known Member

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    Thank you for your reply ! Use the H.O firing order even if my motor is a 74 non h.o block but has the custom grind cam ? Just making sure I got this right !
     
  6. Brandon123

    Brandon123 Active Member

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    Yes, the block has nothing to do with it.. it's all based on how the cam is made and all custom cams are h.o./351w firing order
     
  7. Brandon123

    Brandon123 Active Member

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    1-3-7-2-6-5-4-8
     
  8. 99GreenStang

    99GreenStang Well-Known Member

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    Cool thanks man ! Maybe this will fix my problems ! Do you think that having this order wrong would cause the backfire ? And didn't they change the order to this because of something to do with crank load back in the day and bearings ? Thanks a million god bless you man , no one else online was really clear about the whole firing deal with a custom cam in a non roller block or Non H.O .
     
  9. Brandon123

    Brandon123 Active Member

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    yes it would definitely make it backfire lol I don't know why they do it with the ho firing order, I imagine it's smoother for the engine. Just like a chevy 4-7 swap it just runs smoother but generally a custom sbf cam is grind on the ho firing order so there is no guess work.
     
  10. Brandon123

    Brandon123 Active Member

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    also you need to open the air mixture screws more. take them 1 1/2 turns out from closed then adjust your idle down
     
  11. 99GreenStang

    99GreenStang Well-Known Member

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    Even with 4 corner idle ?? I thought the purpose of 4 corner idle was to minimize adjustment and of course tune for aggressive camshafts offering finer tuning . That seems like a lot but I'll give it a shot ! Also my timing is 8 degrees initial and I've got the silver bushing in which is 25 degrees of advance and the light blue spring and light silver . This sound about right ? Also how much Initial timing do you think is too much ? Do you have a spring recommendation of your own for my setup ? Camshaft is 280 duration and 480 lift with the 4r70w with no stall and obviosouly a msd mechanical advance distributor . Just seeing if you have an idea about that this is my first time doing a mechanical distributor . Also in my previous post I asked if the firing order deal if that would cause the backfire and what are the benefits of using that firing order ? I'm gonna change the wires over tomorrow to the firing order you listed and I hope she works ! I appreciate all your help and I'm sorry for all the questions and long post just trying to make sure you have sufficent information . God bless
     
  12. 99GreenStang

    99GreenStang Well-Known Member

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    Lol sorry I overlooked your answer to my post about would that cause that ! My bad lol !
     
  13. Brandon123

    Brandon123 Active Member

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    the 4 corner screws should be out more.. my small carb recommended 2 1/2 turns out to start with but it was a bigger carb than yours. For reference my 4 corner screws are 4 turns out lol I run very rich at idle because my motor needs it in the upper rpms, but your motor is very mild. I would switch the plug wires around then go drive it.. If it still pops and backfires when you get down on it then open up the 4 corner screws because it is running lean.

    As far as what springs to get? Just contact whoever made your cam.. I would trust them over anyone I talked to on the internet.

    And timing... I have no clue man... I've never really messed with timing on a mild motor to much.. I don't want to give you bad advice. It will depend on your compression, what heads you have (aluminum or cast iron, I don't remember you saying) and what octane fuel you run.
     
  14. Brandon123

    Brandon123 Active Member

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    A quick search on google shows a lot of people using about 12* initial timing and 20* mechanical for a total of about 32*-34* total
     
  15. 99GreenStang

    99GreenStang Well-Known Member

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    4 turns out !!!!! My carb screws don't turn out without potentially falling out over maybe 3 turns heck even the proform double pumper 750 I took of this same motor wouldn't adjust that much lol ! And I was not talking about head springs I am talking about the distrubtor springs as it is mechanical . I believe my timing should be about right I've got 25 degrees advance and 8 initial so that's 33 total timing and I believe I'm full advance at 4.000 rpm . I'm running gt40p heads , flat top pistons , Weiand xcelerator intake , heads have 600 lift springs and fuller roller rockers 1.6 and pump gas . Just to give ya idea . What do you think I will end up close too on amount of turns on the carb ? My understanding is 4 corner idle you turn them less obviously cause there made to fine tune for aggressive cams and radical engines . I appreciate all your info as well man !
     
  16. Brandon123

    Brandon123 Active Member

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    Like I said.. 1/2 turn out doesn't seem like enough to me.. It may idle good but when you get the rpms up it may not have enough fuel to get past 3k rpms Just try to switch the wires to the ho firing order tomorrow and crank it up. It should sound a lot better right away. Then drive it and if it starts popping and backfiring when you try to get the rpms up then take the 4 corner screws out some. Just don't be suprised if 1/2 turn out isn't enough. Same thing I tried.. I turned my screws in, in order to get my solid roller cam to idle nicely.. sounded smooth as a e303 but wouldn't go past 2500 rpms on the street

    And I don't know about the springs.. Try what you have first, seems like it should atleast run smooth on that, then when you get the carb figured out you can play with the timing and see what it likes.. My distributor is locked in at 36* all the time so I don't really mess with it
     
  17. decipha

    decipha Well-Known Member

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    33 degrees is pretty high for gt40p's with flat tops, typically a setup like that will MBT out at about 28 degrees
    yank out some timing and see if you pick up some MPHs
    what do the plugs say?

    I have an mbt reference chart on my website under reference

    8 initial is fine with 28 all in at 3k-3200, +12 on the advance and she should be happy

    tightening the idle screws should lean her out, ideally you want to lean her out until idle start dropping, this is the 'max lean idle' where the engine is most happiest
     
  18. 99GreenStang

    99GreenStang Well-Known Member

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    My distributor is a electronic mechanical distributor ! I don't have vacuum advance . I was told by a summit tech that I need to aim for right around 34-36 degrees total timing , on my dizzy you have to change a stop bushing inside to change how much advance you want the bigger the bushing the less the advance and vice versa . ALSo there is springs the lighter the spring the quicker the advance opens to full advance and vice versa as well on that . From what the post above says if I had 8 initial and 12 degrees advance that would be 20 degrees total timing ? I was told I need around 34-36 with an Initial timing of 8-12 degrees give or take . After this is said I'm not sure if you knew this dizzy wasn't vacuum advance or not .
     
  19. 99GreenStang

    99GreenStang Well-Known Member

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    Also plugs are heavy carbon black but I feel this is because of improper firing and excessive fuel on previous carb so can't really tell much on them at the moment . Maybe they will burn off ??
     
  20. nightfire

    nightfire Well-Known Member

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    Because it's a pain in the ass to do, more money than it's worth, and you would have had far better performance with a mod motor with half the cost? Yeah, that's why.

    :dontknow: